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Question

I'm trying to match the last occurrence of a pattern in a string.

I want to get the last word in the parenthesis in the following string:

(Don't match this) and not this (but this)

I've tried the following,

\s(\((.*?)\))(?!\))

But this matches both occurrences, not only the last one. Is it possible to just match the last one?

Answer 1

Match all strings in bracket /\\(.*?\\)/g and post process the result

You can just match all strings satisfying the pattern, and pick the last element from the resulting array. There is no need to come up with a complicated regex for this problem.

> "(Don't match this) and not this (but this)".match(/\(.*?\)/g).pop()
< "(but this)"

> "(Don't match this) and not this (but this) (more)".match(/\(.*?\)/g).pop()
< "(more)"

> "(Don't match this) and not this (but this) (more) the end".match(/\(.*?\)/g).pop()
< "(more)"

Don't want the () in the result? Just use slice(1, -1) to get rid of them, since the pattern fixes their positions:

> "(Don't match this) and not this (but this)".match(/\(.*?\)/g).pop().slice(1, -1)
< "but this"

> "(Don't match this) and not this (but this) (more) the end".match(/\(.*?\)/g).pop().slice(1, -1)
< "more"

Using .* to search for the last instance of the pattern

This is an alternate solution with simple regex. We make use of the greedy property of .* to search for the furthest instance matching pattern \\((.*?)\\) , where the result is captured into capturing group 1:

/^.*\((.*?)\)/

Note that no global flag is used here. When the regex is non-global (find first match only), match function returns the text captured by capturing group, along with the main match.

> "(Don't match this) and not this (but this)".match(/^.*\((.*?)\)/)[1]
< "but this"

> "(Don't match this) and not this (but this) (more) the end".match(/^.*\((.*?)\)/)[1]
< "more"

The ^ is an optimization to prevent the engine from "bumping along" to search at subsequent indices when the pattern .*\\((.*?)\\) fails to match from index 0.

Answer 2

You can use non capturing parenthesis to consume previous matches:

var string="(Don't match this) and not this (but this) definitely not this";
var last_match=string.match(/(?:.*\()([^\)]*)(?:\)[^\(]*)/)[1];

Tested using web developer console:

< var string="(Don't match this) and not this (but this) definitely not this";
< string.match(/(?:.*\()([^\)]*)(?:\)[^\(]*)/)[1]
>"but this"

Here is the link for testing: https://regex101.com/r/gT5lT5/2
If you want that the enclosing parenthesis to be part of the match, check https://regex101.com/r/gT5lT5/1